The present invention relates to the field of intrusion detection. More particularly, it relates to improvements in intrusion detection systems of the type which passively monitor or "supervise" the operating status of the intrusion sensor components to assure that each sensor is, indeed, functioning properly and, hence, capable of detecting intrusion.
An intrusion detection system in which the various intrusion-sensing elements are non-functioning is, of course, of psychological value only. Obviously, in an detection system, the level of security depends on the percentage of sensors which are functioning at any given time. Since a non-functioning sensor is not easy to detect without actually "walk-testing" the sensor to determine whether it produces an alarm output, it is becoming increasingly common to incorporate a so-called "supervisory" circuit in such systems to monitor the operating status of certain sensor components which are particularly prone to fail, such as the Gunn diode in a microwave detection system. In the event of a component failure, such circuits operate to activate a "supervisory" or "trouble" alarm (e.g., a light-emitting diode) to alert the user of the problem. Detection systems incorporating such supervisory circuits are disclosed, for example, in the commonly assigned U.S. Pat. No. 4,660,024 to R. L. McMaster.
In the commonly assigned U.S. application Ser. No. 492,482, filed on Mar. 12, 1990 in the name of W. S. Dipoala and entitled ACTIVE SUPERVISION OF MOTION DETECTION SYSTEMS, there is disclosed a dual-technology (passive-infrared/microwave) intruder detection system in which both sensor components are "actively" supervised by periodically simulating, within the system, a target of interest. In the event either sensor fails to detect the simulated target, a supervisory alarm is produced. While such "active" supervision provides optimal protection against sensor failure, it does so at the expense of requiring target-simulation apparatus within each sensor device.
Recently, it has become known to "passively" supervise the detection capability of intrusion sensors by monitoring the pedestrian-produced activity of the sensors during those periods when the system is "disarmed", e.g., during the daylight hours in which the protected premises are being used by the owner of the system and the alarm has been inactivated. In a multi-sensor system, the sueprvisory apparatus usually includes a display which indicates which of the several sensors have been activated or "tripped" during the disarm period and, hence, are functional; it also, of course, indicates those which have not been activated. To prevent the system from being re-armed without having the operability of those non-activated sensors verified (e.g., by walk-testing), it is common for the supervisory circuit to inhibit re-arming until it detects that all sensors have been activated. While this arrangement provides a high degree of security, it can be a nuisance to a user who, for example, arms the system after verifying that all sensors are functional and then realizes that he forgot something inside the protected premises. To re-enter such premises, even for a moment, means that he must walk-test all sensors, since there is no intervening traffic to do this job for him. Because of this inconvenience, there is some reluctance on the part of the security customer to opt for this very effective passive supervisory feature.
In the commonly assigned U.S. application Ser. No. 576,055, filed on Aug. 31, 1990 in the names of J. Berube et al., now U.S. Pat. No. 5,057,817 entitled INTRUDER DETECTION SYSTEM WITH PASSIVE SELF-SUPERVISION, there is disclosed an intrusion detection system which overcomes the above-mentioned disadvantage. Such a system is re-armable if either of two conditions prevail, namely, (a) all of the intrusion sensors have been activated during the disarm period, or (b) an attempt to re-arm occurs within a relatively brief, predetermined time interval (e.g., within one hour) immediately following disarming of the system. The latter condition is provided by a programmable timer which provides a continuous signal for a predetermined time interval each time a disarm signal is produced by the system user. Preferably, the timer output, together with the output of the system's supervisory circuit (indicating that all sensors have been activated during the disarm period) serve as the input to a logical OR gate which provides an arm-enabling signal whenever either of its inputs is present. By this arrangement, the system owner/user may re-enter the protected premises after disarming the system and, so long as the system is re-armed within the pre-set period established by the timer, he need not walk-test all sensors prior to rearming.
From any "low security" applications, the requirement that the system user verify the operating status of each intrusion sensor prior to arming the system is too burdensome for system user. Even with the assistance of authorized traffic in the protected region, the owner must still assure that each sensor is functional every time he attempts to arm the system. As indicated above, when the system is armed for only a brief period, many of the sensors in a multisensor system will not be activated by authorized traffic, requiring the user to walk-test all non-activated sensors.